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Macbeth is paranoid of all kinds of things. He is scared he'll be discovered, he is scared he's going to hell.....In Act 4 the Apparition tells Macbeth that to be beware of MacDuff. Then the second apparition states nobody born from a woman will ever harm Macbeth. Macbeth figures he will make sure he does away with Macduff anyway.
"Then live, Macduff. What need I fear of thee?
But yet I’ll make assurance double sure,
And take a bond of fate. Thou shalt not live,
That I may tell pale-hearted fear it lies,
And sleep in spite of thunder." (Act 4)